On Mon, Oct 20, 2025 at 02:22:35PM -0700, Gustavo Luiz Duarte wrote:
> The update_userdata() function constructs the complete userdata string
> in nt->extradata_complete and updates nt->userdata_length. This data
> is then read by write_msg() and write_ext_msg() when sending netconsole
> messages. However, update_userdata() was not holding target_list_lock
> during this process, allowing concurrent message transmission to read
> partially updated userdata.
>
> This race condition could result in netconsole messages containing
> incomplete or inconsistent userdata - for example, reading the old
> userdata_length with new extradata_complete content, or vice versa,
> leading to truncated or corrupted output.
>
> Fix this by acquiring target_list_lock with spin_lock_irqsave() before
> updating extradata_complete and userdata_length, and releasing it after
> both fields are fully updated. This ensures that readers see a
> consistent view of the userdata, preventing corruption during concurrent
> access.
>
> The fix aligns with the existing locking pattern used throughout the
> netconsole code, where target_list_lock protects access to target
> fields including buf[] and msgcounter that are accessed during message
> transmission.
>
> Fixes: df03f830d099 ("net: netconsole: cache userdata formatted string in
> netconsole_target")
nit: no blank line here please
>
> Signed-off-by: Gustavo Luiz Duarte <[email protected]>
> ---
> drivers/net/netconsole.c | 5 +++++
> 1 file changed, 5 insertions(+)
>
> diff --git a/drivers/net/netconsole.c b/drivers/net/netconsole.c
> index 194570443493b..1f9cf6b12dfc5 100644
> --- a/drivers/net/netconsole.c
> +++ b/drivers/net/netconsole.c
> @@ -888,6 +888,9 @@ static void update_userdata(struct netconsole_target *nt)
> {
> int complete_idx = 0, child_count = 0;
> struct list_head *entry;
> + unsigned long flags;
> +
> + spin_lock_irqsave(&target_list_lock, flags);
>
> /* Clear the current string in case the last userdatum was deleted */
> nt->userdata_length = 0;
> @@ -918,6 +921,8 @@ static void update_userdata(struct netconsole_target *nt)
> }
> nt->userdata_length = strnlen(nt->extradata_complete,
> sizeof(nt->extradata_complete));
> +
> + spin_unlock_irqrestore(&target_list_lock, flags);
> }
>
> static ssize_t userdatum_value_store(struct config_item *item, const char
> *buf,
>
> --
> 2.47.3
>
>
